Get the DK2 first and upgrade your box if you need to. The 670 will let you at least try it out. Most of us are running it on budget settings even with our 980GTXes anyway.
Water cooling is a pain in the butt, don't bother. You'll have leaks and destroy something and need to buy fittings and stuff. If you have to ask, just keep your stuff simple stupid and when it breaks you'll know how to fix it instead of having a pile of wet circuit boards.
Sounds like a good plan, especially since I should be able to use the Rift for non-Elite stuff as well. I still have almost a month until my next paycheck, but I'm leaning more and more in this direction.
Thanks!